Elemental Shift: Periodic Table Gets Weight Changes

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Ten elements that help make up the universe, including the carbon
our biology is based on and the oxygen in the air we breathe, are
now getting changed in an unprecedented way — they are getting
their very atomic weights altered.

Scientists have not invented some magical way to transform the
masses of all these elements. Instead, they are updating what are
often thought of as constants of nature on the periodic table.

"For more than a century-and–a-half, many were taught to use
standard atomic weights — a single value — found on the inside
cover of chemistry
textbooks and on the periodic table of the elements," said
physicist Michael Wieser at the University of Calgary. "As
technology improved, we have discovered that the numbers on our
chart are not as static as we have previously believed."

The standard
atomic weight of an element, which is made up of one type of
atom, is based on the mass of its atoms. The problem that
scientists are now addressing is rooted in the fact that these
atoms don't always have the same masses. While all the atoms
making up an element have the same number of protons, elements
have variants known as isotopes that possess different numbers of
neutrons in their nuclei, making some lighter or heavier than
others.

Certain elements have more than one stable isotope. For instance,
carbon has two — carbon-12 and carbon-13. (The numbers in each
isotope reveal how many particles they have in their nuclei —
carbon-12 has six protons and six neutrons.) In the past, to give
a standard atomic weight for these elements, scientists averaged
out the atomic weights of these isotopes based on
how common those isotopes are — the more plentiful an isotope
was, the bigger a role it played in the standard atomic weight.

However, the abundance of an isotope can vary in nature, leading
to variations in an element's atomic weight. For example, sulfur
is commonly known to have a standard atomic weight of 32.065, but
its real atomic weight can be anywhere between 32.059 and 32.076,
depending on where the element is found.

These small variations in an element's atomic weight can weigh
heavily on research and industry. For example, precise
measurements of the abundances of carbon isotopes are used to
determine purity and source of food, such as honey and vanilla.
Isotopic measurements of nitrogen, chlorine and other elements
help trace pollutants in streams and groundwater. In sports
doping investigations, scientists can identify
performance-enhancing testosterone in the human body because
the atomic weight of carbon in natural human testosterone is
higher than that in pharmaceutical testosterone.

"There's a lot of practical information we can get from knowing
atomic weight, all these significant problems and issues where
knowing atomic isotope abundance can play a key role," Wieser
told LiveScience. He serves as secretary of the International
Union of Pure and Applied Chemistry's (IUPAC) Commission on
Isotopic Abundances and Atomic Weights, which oversees the
evaluation and dissemination of atomic-weight values.

Now, for the first time in history, the standard atomic weights
of 10 elements — hydrogen, lithium, boron, carbon, nitrogen,
oxygen, silicon, sulfur, chlorine and thallium — will get
expressed in a new way that will more accurately reflect how
these elements are found in nature. Instead of single values,
they will get expressed as intervals, having upper and lower
bounds, to more accurately convey variations in atomic weight.
For instance, carbon's standard atomic weight is listed as an
interval between 12.0096 and 12.0116.

The other elements on the periodic table remain the same, as
elements with just one stable isotope do not show variations in
their atomic weights. For example, the standard atomic weights
for fluorine, aluminum, sodium and gold are constant, and their
values are known to more than six decimal places.

These changes might seem confusing to students and scientists.
Which number should they use on a test, or in the lab?
Ultimately, it will depend on the element and the context.

If they just want to perform a simple calculation involving these
10 elements, they can use a single value called a conventional
atomic weight, Wieser said. If they need more precision — more
decimal places in the number — they can look up an atomic-weight
value for the specific context they have in mind. For instance,
"boron in seawater has a very narrow atomic-weight range, so I
could select a value of 10.818," research chemist Tyler Coplen,
director of the U.S. Geological Survey's Reston Stable Isotope
Laboratory, who was worked on these changes for the past 15
years, told LiveScience.

Coplen and Wieser said they were completely surprised about the
attention this change has received.

"People might remember sitting in a chemistry class with the
periodic table hanging on the wall, and after seeing that some
elements such as sodium or gold were measured to
an incredible precision, wondered why others such as sulfur and
lead weren't measured to the same precision," Wieser said. "Now
this change might answer that."

These changes became official when IUPAC published them online
Dec. 12 in the journal Pure and Applied Chemistry.